Goossens and colleagues developed their new model of the Moon's interior, to explain anomalies in the moon's orbit and its gravitational readings.

"Something solid rotates slightly differently to something with liquids inside it, and those observations were giving us clues about the interior of the Moon," says Goossens.

Previous seismic data had hinted at a liquid region around the Moon's core, but the data hasn't always been clear, and other studies using the same information, had produced models without the inclusion of a liquid lower mantle layer.

"This has always been an issue of discussion," says Goossens.

"But our model's inclusion of a liquid layer in the lower mantle provides a far better fit for all the observations," he says.

Computer model

The authors developed a numerical computer model of the lunar interior which included a low viscosity, molten layer at the mantle-core boundary.

The researchers ran their model through a series of simulations which included lunar rotational cycles matching the Moon's orbit around the Earth, and the Earth-Moon system's yearly orbit around the Sun.

Just as the Moon pulls on the Earth to generate high and low tides twice a day, the Earth and Sun also pull on the Moon, and this can generate enough friction to melt rocks deep inside the lunar mantle, say the researchers.

"Our modelling indicates a liquid layer between a radius of about 350 and 500 kilometres from the centre of the Moon at the base of the mantle," says Goossens.

"We found the viscosity was remarkably low, so it's more liquid than you would normally think.

If the modelling is the correct, the presence of a liquid layer around the lunar core has implications for the history and evolution of the Moon.

"The Moon is relatively small and so, was thought to have cooled and solidified all the way though relatively quickly to become solid rock," says Goossens.

"We need to look further into how you can maintain such a soft layer over such a long history [4.5 billion years since the giant impact which created the Earth-Moon system].

"We think ongoing tidal heating from the Earth is playing a part in keeping this layer liquid. There's a balance you get between tidal heating from the Earth and convection from dissipating heat."